The present invention relates to a vortex flow meter for measuring the flow velocity or quantity of a fluid by utilizing Karman's vortices. More particularly, this invention relates to such a meter equipped with a piezooelectric sensor and a charge amplifier.
It is known that when an object is placed in a fluid, vortices are generated on the sides of the object alternately and regularly to form a flow of vortex street in the downstream. This is referred to as a Karman's vortex street, and the number of vortices (vortex frequency) generated during a unit time is proportional to the flow velocity of the fluid.
The vortex flow meter has a vortex generator disposed in a duct carrying the fluid to be measured, and Karman's vortices proportional to the flow velocity are generated therefrom. These vortices are detected by means of a sensor such as a thermosensitive element or a piezoelectric element to produce an electrical signal corresponding to the flow velocity or quantity of the fluid. One vortex flow meter employing a piezoelectric element to detect the fluid vibration as a change in an AC voltage is disclosed in the U.S. Pat. No. 3,948,098.
The piezoelectric sensor is further capable of detecting the fluid vibration as a change in the electric charge quantity. In this case, the charge quantity obtained from the piezoelectric sensor is converted into a voltage signal by means of a charge amplifier, of which the cutoff frequency normally is selected to be below the minimum value (1 Hz) of the vortex frequency to be measured so that a satisfactory response characteristic is attained within the vortex frequency range (approximately from 1 Hz to 120 Hz when the fluid to be measured is a liquid). For ensuring excellent low-range characteristics in the charge amplifier, it is necessary to select a large value for the time constant of a resistor-capacitor feedback circuit for the amplifier. However, the sensitivity of the amplifier is dependent on the value of the capacitor, which should be reduced to achieve a high sensitivity. Therefore, increasing the resistance is required to attain a large time constant. For example, when setting the cutoff frequency of the charge amplifier to 1 Hz, the resistance required becomes extremely high, e.g. above 1,000 megohms. A problem arises with respect to the reliability in the resistance of any value exceeding 1,000 megohms, in addition to a disadvantage of high production cost. Thus, practical use has not yet been achieved for a vortex flow meter of the type that performs signal processing after detecting the fluid vibration as a change in the electric charge quantity by a piezoelectric sensor.
A noise component referred to as fluctuation of frequencies lower than the vortex frequencies (ranging from 1 to 120 Hz) is superposed on the vortex signal. The noise frequency becomes higher with increasing vortex frequency, and its magnitude also increases with the frequency. Moreover, when detecting the vortex signal by a piezoelectric sensor, detection is affected by noise such as duct vibration caused by a pump or the like. The noise frequencies resulting from duct vibration are within a range from several ten hertz to several hundred hertz, and the magnitude thereof increases generally in proportion to the frequency. In a vortex flow meter, it is desired that any harmful effect of such noise components be effectively eliminated to achieve adequate detection of the vortex signal at a satisfactory signal-to-noise ratio throughout a wide range of flow velocities.